1. Field of the Invention
The present invention relates to a hub and, more particularly, to a rear hub for a bicycle.
2. Description of the Related Art
A conventional rear hub for a bicycle in accordance with the prior art shown in FIGS. 4-6 comprises a mandrel 20 having two ends each formed with a threaded section 21, a hub body 10 rotatably mounted on the mandrel 20 and having a side formed with a plurality of ratchet teeth 12, a freewheel 30 rotatably mounted on the mandrel 20 and having a first end provided with a oneway ratchet wheel 32 meshing with the ratchet teeth 12 of the hub body 10 to rotate the hub body 10 in a oneway direction only and a second end provided with a driven gear 33 meshing with a chain which meshes with a chainwheel, two locking nuts 22 each screwed onto the respective threaded section 21 of the mandrel 20 to limit the hub body 10 and the freewheel 30, two first bearings 42 mounted between the mandrel 20 and the hub body 10, and a plurality of second bearings 40 mounted between the mandrel 20 and the freewheel 30. The hub body 10 has an inside formed with a shaft hole 11 to allow passage of the mandrel 20. The freewheel 30 has an inside formed with a through hole 31 to allow passage of the second bearing 40. Thus, the chainwheel drives the chain to rotate the driven gear 33 which rotates the ratchet wheel 32 to rotate the hub body 10 in a oneway direction only.
However, the freewheel 30 has a diameter corresponding to that of the freewheel 30 so that the freewheel 30 is made to have a greater diameter to increase the tooth number of the driven gear 33 to increase the tooth number of the chainwheel to increase the diameter of the chainwheel so as to increase the whole volume and weight of the bicycle. In addition, the freewheel 30 and the chainwheel have a greater diameter and tooth number so that the chainwheel easily touches the ground when the bicycle is disposed at an inclined state during an acrobatic performance, thereby causing danger to the rider. Further, the mandrel 20 is made of a solid metallic material so that the mandrel 20 has a heavier weight, thereby increasing the whole weight of the bicycle.